libc: Add strcasecmp support

Add strcasecmp() for case insensitive string compares. There is no ISO
standard function for this purpose but strcasecmp() is POSIX standard.

Backported from upstream lk commit 9ba1f165cdf947e ("[libc][string] Add strcasecmp support")
without adding to the "string.h" header as POSIX says it's in "strings.h"
which we already have for whatever reason.

Author: TravMurav

lib/libc/string/rules.mk

@@ -9,6 +9,7 @@ C_STRING_OPS := \
 	memmove \
 	memset \
 	memscpy \
+	strcasecmp \
 	strcat \
 	strchr \
 	strcmp \

lib/libc/string/strcasecmp.c

@@ -0,0 +1,28 @@
+/*
+** Copyright 2001, Travis Geiselbrecht. All rights reserved.
+** Distributed under the terms of the NewOS License.
+*/
+/*
+ * Copyright (c) 2008 Travis Geiselbrecht
+ * Copyright 2022 Google LLC
+ *
+ * Use of this source code is governed by a MIT-style
+ * license that can be found in the LICENSE file or at
+ * https://opensource.org/licenses/MIT
+ */
+#include <ctype.h>
+#include <strings.h>
+#include <sys/types.h>
+
+int strcasecmp(char const *cs, char const *ct) {
+    signed char __res;
+
+    while (1) {
+        if ((__res = tolower(*cs) - tolower(*ct)) != 0 || !*cs)
+            break;
+        cs++;
+        ct++;
+    }
+
+    return __res;
+}